Non-contact (irradiative) heating is needed in the deposition of thin films onto a moving substrate in the absence of convective (gas) heating. It is customary to employ a cavity-type heater where a substrate is disposed in the cavity while the walls of the cavity-type heater are heated to a predetermined temperature T1. In an ideal cavity, due to radiation exchange, the substrate may reach a temperature Ts which is close to the temperature of the cavity walls, i.e., (Ts≈T1). The temperature Ts of the heated substrate in the ideal cavity is theoretically uniform over the surface of the substrate and is stable.
In reality, however, the cavity walls have at least one opening formed therethrough to allow a deposition flux generated at the remote source of material into the cavity to reach the substrate for material deposition thereon.
Due to the presence of the opening in the walls of the cavity, a portion of the thermal energy of the substrate escapes from the cavity, thus causing a decrease of the substrate temperature Ts below the temperature T1 of the walls of the cavity (Ts<T1).
In a non-ideal cavity, the temperature of the substrate will always be lower than the temperature of the walls. This causes unwanted obstacles in the deposition of high quality films of multi-component materials. Specifically, in the deposition of crystalline epitaxially grown films of a material, the surface of growth; e.g., substrate surface, must be heated to an optimal temperature that is close to but less than the temperature of decomposition Td of the material (Td≈Ts).
Thus, since Ts<T1 and Td≈Ts, the wall temperature T1 is higher than the decomposition temperature Td (T1>Td). Under this condition, a fraction of the material passing into the cavity with the deposition flux (and unavoidably reaching the cavity walls) will decompose along with the material reaching the substrate. The re-evaporation of the material components from the walls will change the composition of the film growing on the substrate and, as a result, may degrade its properties.
Another technique customarily used to heat a substrate is radiation heating wherein a radiation from a remote source having an effective temperature T2>>Ts, is directed onto the substrate. A laser or quartz halogen lamp may be used as the source of the radiation. Disadvantageously, the laser radiation of sufficient power (˜0.5 kW for the substrates of reasonably large area ˜10 cm2) is relatively expensive to produce, and it is difficult to attain a uniform and stable temperature Ts over the substrate area in the open heater arrangement.
The heater arrangements using lamps, in addition to high cost, require the lamps to be protected against the unwanted deposit of material. To lower the material deposited on the lamp surface, the lamps are usually distanced from the substrate, which requires radiation delivery optics, including focusing elements, mirrors, etc., resulting in further degradation of the uniformity and stability of the substrate temperature.
A technique for non-contact heating of a substrate for material deposition which provides stable and uniform heating of the substrate surface to a temperature satisfying the conditions required for multi-component films deposition is therefore needed in the industry.